Electrets and polymer electrets in particular have important device applications in current and future products in the electronics and computer industry. They have been used in such devices as capacitors and electret microphones, piezoelectric electrets in loudspeakers, pressure sensors and actuators, pyroelectric electrets in laser power meters and temperature sensors. In order for these dielectrics to have ferroelectric, piezoelectric, pyroelectric or nonlinear optical properties, they must be poled in an intense electric field of the order of 1 Megavolt/cm to uniformly align their electric dipoles. This poling is currently done in one of three ways, by a corona discharge, by uniaxially or biaxially stretching for polymer films or by applying a large voltage across the electret while it is immersed in a high dielectric constant oil. Corona poling is accomplished by using a field emission tip or wire to produce ionized gas molecules which are deposited on the front surface of an electret sheet which is metallized on its back side. With a surface potential of the order of 1000 Volts on an electret with a typical thickness of tens of micrometers, a poling field of approximately 1 Megavolts/cm can be achieved. Problems with corona poling include the use of high voltages at high currents, considerable chemical reactivity due to the ionized species produced, possible sputtering away of the corona tip or wire and gas evolution generated from chemical reaction. Stretching for polymer films can also enhance the orientation of the polymer chains and is often done when the polymer is extruded in sheets and may be done at elevated temperatures or in conjunction with applied electric fields. Obviously stretching cannot be done for crystalline electrets or for polymers on a rigid substrate and for those cases where it can be done, there are considerable changes in the film thickness and roughening of the film's surface. Finally, electrodes may be deposited on both sided of the electret film and a large voltage may be applied across the film while it is immersed in a bath of high dielectric constant oil. This is necessary in order to avoid dielectric breakdown which would occur in air at fields of the order of 30 Kilovolts/cm. Difficulties with this procedure include contamination of the electret with the oil or impurities, the need for both sides to have electrodes and incompatibility with certain applications of polymer electrets such as RAM memories.
Whitmore, U.S. Pat. No. 3,396,308, discloses a device for generating a flow of ionized gas that is directed toward and against the surface of a dielectric web for altering or changing the electrostatic charge on the web.
Roth et al., U.S. Pat. No. 3,549,962, discloses a method for uniformly charging insulating surfaces by providing a surface having a non-uniform charge density thereon and contacting the non-uniformly charged surface with a material causing the charged surface to attain a uniform charge density in contacted areas.
Hawkins, U.S. Pat. No. 3,686,374, discloses a method for increasing the effectiveness of electrostatic pinning of a dielectric film onto an electrically grounded moving surface by passing the film in proximity to but out of contact with at least one electrode. This imparts an electrostatic charge to the surface of the film. In addition, the electrode is surrounded with an atmosphere consisting essentially of a gas in which a wire current before breakdown of at least about 100 microampere/inch of wire can be generated.
Adler, U.S. Pat. No. 3,761,746, discloses poling a ferro-electric body using an ionized gas or other medium as a poling electrode.
Kamogawa et al., U.S. Pat. No. 4,021,709, discloses a method for enhancing the charge retention of a photosensitive layer wherein an ion wind is created and applied to the photosensitive material prior to its reuse.
Garbett, U.S. Pat. No. 4,213,168 discloses a dielectric tape of polyfluoroethylenepropylene which is electrostatically charged to a high level charge density using a motor driven capstan on which the tape is moved at a controlled velocity past a sponge assembly charged by a high DC voltage. The sponge assembly is wetted with a dielectric fluid mixture of methyl alcohol, ethyl alcohol and acetone. By endosmosis, a bead of fluid from between the sponge and the FEP tape causes the fluid to be evenly distributed on the tape.
Talbott et al., U.S. Pat. No. 4,664,856, discloses a method for treating discrete pieces of material to ensure adequate conductance thereof for orientation in an electric field for electrostatic alignment. The material is exposed to a conductance-improving substance or substances, such as an alkali or alkaline earth metal salt of a weak acid, quaternary ammonium salts, or gases such as ammonia, chlorine, sulfur dioxide, etc. prior to being subjected to the electric field.
Micheron et al., U.S. Pat. No. 4,734,228 discloses a method for preparing a material having piezoelectric properties by dipole orientation of a poled dielectric. A dipole polarization is effected by a charged jet of small liquid drops or droplets. An electric field is created between two superficial zones of an object brought to different electric potentials. At least one of the zones is exposed to an electrically charged jet of droplets.
Bergen, U.S. Pat. No. 4,762,997, discloses a method for charging a receptor surface to a predetermined voltage by generating ions in a chamber, entraining the ions in a rapidly moving fluid stream passing into, through and out of the chamber, depositing the ions on a charge receptor and biasing the back of the charge receptor with a bias equal to and of opposite potential of said predetermined voltage desired on the receptor surface.